1. Field of the Invention
This invention generally relates to a device for developing an electrostatic latent image formed on an imaging member by application of a thin film of developer charged to a predetermined polarity, and, in particular, to a device for forming a developer film for use in developing such an electrostatic latent image. More specifically, the present invention is concerned with a device for forming a film of magnetically attractable toner to be used in developing an electrostatic latent image formed on an image bearing member.
2. Description of the Prior Art
In a developing device employing magnetically attractable toner, or so-called single component toner, it is required to form a thin film, uniform in thickness, of such toner which is sufficiently charged to a predetermined polarity. For this reason, as shown in FIG. 1, inside of a developing sleeve 2, which is driven to rotate counterclockwise, there is disposed a magnet roll 3, which is also driven to rotate counterclockwise in the illustrated example, and magnetically attractable toner 1 is carried on the outer peripheral surface of the developing sleeve 2 as attracted thereto by the magnetic forces emanating from the magnet roll 3. In this structure, there is also provided a blade 4, which is comprised of an elastic, magnetic material, with its free end pressed against the outer peripheral surface of the developing sleeve 2 due to magnetic attractive forces applied by the magnet roll 3. Thus, when the toner 1 carried on the developing sleeve 2 is brought to a gap P defined between the free end of the blade 4 and the outer peripheral surface of sleeve 2, the toner 1 becomes pressed between the blade 4 and the sleeve 2 thereby causing the toner 1 to be formed into a thin film uniform in thickness. At the same time, since the toner 1 is subjected to friction against the blade 4 and the sleeve 2, the toner 1 becomes triboelectrically charged to a predetermined polarity as being formed into a thin film.
In this case, the pressing force between the blade 4 and the sleeve 2 fluctuates cyclically with the rotation of the magnet roll 3. Under the condition, if the pressing force increases excessively for some reason, then there is produced an significantly large frictional force which could then suddenly increase the temperature of the toner 1 between the blade 4 and the sleeve 2. In this case, the toner 1 thus heated could melt and become fixedly attached to the blade 4. If the toner 1 is fixedly attached locally on the blade 4, it is impossible to form a thin film of toner uniform in thickness and, often times, streaks are formed in the resulting thin film of toner. This is disadvantageous because such streaks will appear in a developed image thereby impairing the developing performance.